Fullstory basically lets me sit behind the shoulder of every user and e.g. watch them use the app in a way I didn't think of, or stumble over the features that I thought were obvious but turns out are very counter-intuitive.

It's totally a game changer for UX design.

Heap, Google Analytics, etc are actually not as useful; I find the app's DB and server logs provide more relevant data. So I don't think I'll add them back.